1. Field of the Invention
The present invention relates generally to perovskite-type catalysts which are useful in carbon monoxide oxidation, hydrocarbon oxidation, nitrogen oxide reduction and oxidation of trapped soot particles. In addition, the present invention relates to perovskite-type materials displaying so-called giant magnetoresistance (GMR). Furthermore, the present invention relates to methods of making and using perovskite-type catalysts and materials.
2. Description of Related Art
Perovskite compositions are nominally designated as ABO3, in which A represents a rare earth metal such as lanthanum, neodymium, cerium or the like, and B represents a transition metal such as cobalt, iron, nickel or the like. It is known in the art that perovskite-type materials are useful for the catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions. It is also known that perovskite materials (powders, single crystals and thin films) containing Mn on the B-site show giant magnetoresistance effect (GMR), such that on application of a magnetic field, the electrical resistivity of the material drops drastically due to a field-induced switching of the crystal structure. For this reason, GMR has attracted considerable attention for device applications, such as magnetic recording heads.
Several techniques have been used to produce perovskite-type catalyst materials for the treatment of exhaust gases from internal combustion engines. The ability of such materials to effectively treat internal combustion exhaust gases depends on the three-way activity of the material, i.e., the capability for nitrogen oxide reduction, carbon monoxide oxidation and unsaturated and saturated hydrocarbon oxidation. The following patents describe such materials and techniques in the three-way catalytic application: U.S. Pat. Nos. 3,865,752; 3,865,923; 3,884,837; 3,897,367; 3,929,670; 4,001,371; 4,049,583; 4,107,163; 4,126,580; 5,318,937. In particular, Remeika in U.S. Pat. No. 3,865,752 describes the use of perovskite phases incorporating Cr or Mn on the B-site of the structure showing high catalytic activity. Lauder teaches in U.S. Pat. No. 4,049,583 (and U.S. Pat. No. 3,897,367) the formation of single-phase perovskite materials showing good activity for CO oxidation and NO reduction. Tabata in U.S. Pat. No. 4,748,143 teaches the production of single-phase perovskite oxidation catalysts where the surface atomic ratio of the mixed rare earth elements and the transition metal is in the range of 1.0:1.0 to 1.1:1.0. The rare-earth component can be introduced using a mixed rare-earth source called “Lex 70” which has a very low Ce content. Tabata further teaches in U.S. Pat. No. 5,185,311 the support of Pd/Fe by perovskites, together with bulk ceria and alumina, as an oxidation catalyst. The perovskite is comprised of rare earths on the A-site and transition metals on the B-site in the ratio 1:1.
In addition to these patents there are numerous studies reported in the scientific literature relating to the fabrication and application of perovskite-type oxide materials in the treatment of internal combustion exhaust emissions. These references include Marcilly et al., J. Am. Ceram. Soc., 53 (1970) 56; Tseung et al., J. Mater. Sci., 5 (1970) 604; Libby, Science, 171 (1971) 449; Voorhoeve et al., Science, 177 (1972) 353; Voorhoeve et al., Science, 180 (1973); Johnson et al., Thermochimica Acta, 7 (1973) 303; Voorhoeve et al., Mat. Res. Bull., 9 (1974) 655; Johnson et al., Ceramic Bulletin, 55 (1976) 520; Voorhoeve et al., Science, 195 (1977) 827; Baythoun et al., J. Mat. Sci., 17 (1982) 2757; Chakraborty et al., J. Mat. Res., 9 (1994) 986. Much of this literature and the patent literature frequently mention that the A-site of the perovskite compound can be occupied by any one of a number of lanthanide elements (e.g. Sakaguchi et al., Electrochimica Acta, 35 (1990) 65). In all these cases, the preparation of the final compound utilizes a single lanthanide, e.g. La2O3. Meadowcroft, in Nature, 226 (1970) 847, refers to the possibility of using a mixed lanthanide source for the preparation of a low-cost perovskite material for use in an oxygen evolution/reduction electrode. U.S. Pat. No. 4,748,143 refers to the use of an ore containing a plurality of rare-earth elements in the form of oxides for making oxidation catalysts.
In addition to the above-mentioned techniques, other techniques have been developed for the production of perovskite materials containing Mn on the B-site which show giant magnetoresistance effect (GMR). Such materials are generally made in the forms of powders, single crystals, and thin films. A common technique is the growth of single-crystal from a phase-pure perovskite source (see, for example, Asamitsu in Nature, 373 (1995) 407). All such techniques use a phase-pure perovskite compound with a single lanthanide on the A-site, in addition to an alkaline earth dopant. An example of such phase-pure perovskite compounds is La1-xSrxMnO3.
It is also known in the art that it is difficult and expensive to prepare individual rare-earth compounds such as individual lanthanides. The cost is high for making perovskite-type materials with a single lanthanide on the A-site. Therefore, a need exists for using low-cost starting materials to manufacture inexpensive catalyst materials, simultaneously having high temperature stability and high three-way activity for use in conversion of CO, hydrocarbons and oxides of nitrogen in modern gasoline-powered automobiles. A need also exists for the manufacture of bulk materials, thin films and single-crystals of materials showing GMR, using inexpensive starting materials.